Resumo. Changes in the horizontal structure of a fragment of Mixed Rain Forest, located at Irati National Forest, in Parana state, were evaluated by measurements performed during the years 2002, 2005 and 2008. The study area has 25 ha, divided into 25 permanent sample plots of 1 ha (100 m x 100 m) each. All trees with diameter at 1,30 m above the ground equal or higher 10 cm were measured and identified. Density, dominance, frequency, importance value and cover value were analyzed and, to understand better the phytosociological estimators and successional stage of the forest, the species were classified into ecophysiological groups: pioneers, early secondary, late secondary and undetermined. There was a decrease of 2.3% in the number of individuals per hectare, with 581 recorded in 2002 and 567 individuals/ha in 2008. Currently, Ilex paraguariensis was the species with the greatest number of individuals, representing 9.4% of the total. The dominance (basal area) changed from 28.6 in 2002 to 30.2 m²/ha in 2008, i.e., there was an increase of 5.1% in the period, and currently Araucaria angustifolia has been the species that dominates the forest with 25.9%. The frequency pointed to an uneven distribution of species in the forest, with rates of 47,4% in 2002 and 46,3% in 2008. Araucaria angustifolia was the dominant species and the most ecologically important one in the period with 35.2 (11.7%) of the total absolute importance value in 2008. According to the phytosociological estimators and ecophysiological groups the forest still has a growing stock, due to the increment in trees with bigger diameters from species such as Araucaria angustifolia and Ocotea porosa , and is in advanced stage of ecological succession, with the majority of species and individuals belonging to the early and late secondary, and every year it becomes more mature with the ingress of new species and shade tolerant individuals.